1. Field of the Invention
This invention relates to tanks for the storage and carrying of fluids and, more particularly, to high packing density, low-cost external fuel tanks for aircraft.
2. Description of the Related Art
Military aircraft operations frequently require much uploading and downloading of fuel and munitions. During such operations, it is important to (a) minimize the time required to prepare aircraft for operations and to service aircraft that have landed so that they are again flight-ready and (b) minimize the potential for a fuel fire hazard from fuel vapor ignition. Military aircraft operations frequently utilize external fuel tanks in order to extend the range and flying time of aircraft. Such aircraft operations require external fuel tanks that may be easily and safely transported and that can withstand some degree of ballistic impact (gunfire), physical impacts (crash), and internal fuel ignition (explosion) without rupture, also known as having limited survivability.
External fuel tanks that are constructed of metal and that may be disassembled for economical shipping or storage when not in use are referred to as "nestable" fuel tanks because, when disassembled, the parts making up individual fuel tanks may be stacked or nested within each other in crates and therefore have a high packing density. Typically, metal nestable fuel tanks have a packing density ratio of from 4:1 to 10:1. That is, the parts making up from 4 to 10 fuel tanks may be stored in nested condition in the same amount of space (cubic feet) required for the packing crate of one fully assembled fuel tank. Unfortunately, metal nestable external fuel tanks may require from four to twelve man-hours or more to assemble.
Typically, a nestable fuel tank may be separated into nose and tail cone sections and a generally cylindrical center section. The nose and tail sections may be removed from the center section by removing a clamping seal ring and may then be stacked, one inside another. The clamping seal ring may be a separate part of the center section or may be an integral part, being welded to the center section. The center section is typically a cylinder that is bolted and sealed by a longitudinal clamp bar and that may be opened for disassembly sufficiently to allow other tank center sections to be inserted inside. Longitudinally spaced frame sections are overwrapped on the circumference by the center section metallic skin of the tank to complete the assembly of the major components of a typical nestable fuel tank.
More importantly, metal nestable external fuel tanks are not particularly impact resistant and easily break apart even from minor impacts, creating an extreme fuel fire hazard. Furthermore, metal nestable external fuel tanks typically explode when subjected to fuel vapor ignition from an electrical component short circuit, static electric discharge, lightning strike or ballistic impact from an incendiary round of ammunition. The most serious deficiency in a metal fuel tank is "hydraulic ram" failure from ballistic impact when the tank is full of fuel. Typically, even small arms fire from thirty caliber rounds will catastrophically rupture a standard metal tank, blowing the tank apart as in an explosion. Standard aluminum nestable fuel tanks are not particularly resistant to fuel fires. For example, aluminum fuel tanks will only last for approximately two to three minutes before melting open or rupturing in a fuel fire.
External fuel tanks constructed of high strength composite plastic materials using filament winding techniques are not nestable but are extremely resistant to crash impact, internal explosions from fuel vapor ignition, ballistic impact, and external fuel fires. Such tanks are typically comprised of a filament-wound fiberglass material defining inner and outer layers with a rigid structural plastic foam or honeycomb material placed between the layers. These high strength composite materials are usually applied onto a thin, leak-tight metallic or plastic liner. The fiberglass material is typically impregnated with a thermosetting plastic resin material. These tanks typically survive ballistic impacts from rounds in excess of fifty caliber with only modest amounts of leakage and will not rupture when exposed to hydraulic ram effects from ballistic impact on a fully fueled tank. These tanks also remain leak-free, retain their structural strength, and will not rupture even when subjected to internal explosions. These plastic, sandwich construction tanks have been able to maintain their containment structure when crash impacted onto concrete from a height of twenty-five feet and with a forward velocity of forty-five feet per second (thirty miles an hour), where a metal nestable tank will only withstand a vertical drop of four to six feet with no forward velocity. The plastic tanks have survived intense external fuel fires for more than ten minutes, and have suffered only superficial damage when subjected to a forced ejection while being suspended from an aircraft. All of these conditions, except forced ejection, would destroy a typical metal nestable fuel tank. Thus, although plastic tanks cannot be disassembled for compact storage, they survive under conditions that would destroy ordinary metal nestable fuel tanks.
The compactness of metal nestable fuel tanks is a very desirable feature for military aircraft operations. Compactness is especially important in transportation of tanks and most importantly in aircraft carrier operations, where all equipment must be kept on board the ship and space is at a premium. Military operations also place extreme importance on safety and survivability. Thus, there is a need for external fuel tanks with the safety and survivability features of plastic tanks that also have the high packing density of metal nestable tanks.